This invention relates to controlling the movement of conditioned air to individual zones of a variable air volume system. In particular, this invention relates to the control of a fan which moves the conditioned air through the ducts of a variable air volume system.
The air supply fan within a variable air volume system, hereinafter referred to as a "VAV system" moves conditioned air down a supply duct to individual zones which are to be heated or cooled by the conditioned air. In order to move the air to these zones, the supply fan must generate sufficient pressure in the supply duct to overcome frictional losses experienced in moving the air to the zones. These frictional losses will vary depending on the level of demand for conditioned air by the individual zones.
To ensure that sufficient pressure is available at all times, a maximum fan discharge pressure requirement is typically established for the supply fan of a variable air volume system. This fan discharge pressure requirement is premised on the fan producing enough pressure in the supply duct to meet the maximum individual air flow requirements of the respective zones. The thus established fan discharge pressure requirement is generally known as a "static pressure set point" for the VAV system. This static pressure set point may be used to, for instance, control the speed of the supply fan. In this regard, a controller for the fan typically requires a variable speed fan motor to either speed up or slow down so as to meet the static pressure set point. It is to be understood that other types of fan control may also be used to meet the required static pressure setpoint.
It is to be appreciated that the supply fan need not always be required to generate the fan discharge pressure defined by the aforementioned static pressure set point. In this regard, there are many air flow or load conditions within the individual zones that do not require maximum air flow. When this occurs, the dampers within the VAV system must modulate the amount of air flow to the respective zones. This produces an increased pressure drop within the supply duct of the VAV system in order to achieve these modulated air flow rates. Conversely, the friction losses within the VAV system decrease exponentially as these individual VAV dampers throttle down the amount of air flow to the individual zones. These frictional and damper pressure drop losses impact the efficiency of the VAV system. It is therefore to be appreciated that under any condition other than full load for the system, the fan will be supplying an excessive amount of pressure which must be offset by closing dampers in order to reduce the air flow to the respective zones.
It is an object of the invention to provide a fan control for a VAV system which does not always require that discharge duct pressure be maintained at an artificially high static pressure set point.
It is another object of the invention to provide a fan control which can adjust the fan discharge pressure to an appropriate level dictated by any load condition of the VAV system.